The present invention relates generally to electric power distribution systems and, more particularly, to an inexpensive and reliable method and apparatus for monitoring current in a neutral circuit for three-phase four-wire electric power distribution systems which detects harmonic currents flowing in the neutral circuit wherein harmonic currents would otherwise go undetected and cause damage to the power distribution system.
There is a danger of undetected and unprotected distribution circuit overloads in buildings due to the increasing use of office automation equipment such as personal computers, computer terminals, facsimile machines, and xerographic copiers. Such office automation equipment is powered by simply plugging the equipment into familiar power outlets within the building. These overloads are caused by harmonic currents generated by power supplies within the equipment. Unfortunately, a building manager normally has little knowledge or control of what equipment is plugged into the power distribution system and such equipment can vary greatly over time. A properly loaded power distribution system today can change to an overloaded system tomorrow without the manager's knowledge. Even with conventional approved wiring practices, power distribution equipment failure can result and worse still, a fire safety problem can be produced.
The problem arises in buildings with three-phase four-wire power distribution systems such as 208Y/120V system. In a typical building, the majority of the power is distributed via single phase 120 volt (120V) branch circuits which are connected to the power outlets. If a shared neutral conductor is used for a plurality of single phase branch circuits, a potential for a severe overload condition exists for that conductor.
The primary winding of the distribution transformer, if the transformer is delta/wye, is also subject to overload when powering the noted office automation equipment even though the secondary phases powering the single phase branch circuits are not overloaded. Overloading of the primary winding is due to circulating 3rd harmonics and multiples of the 3rd harmonics which occur when the primary winding of the distribution transformer is wired in a delta configuration, which is usually the case.
The harmonic currents which create the overload problems of the neutral conductors are generated by power supplies which use full wave bridge rectifiers tightly coupled to relatively large filter and energy storage shunt capacitors in the power supplies. The capacitors remain charged to near the peak of the applied ac voltage and accordingly draw recharging current only during the narrow periods of each half cycle of applied ac voltage when that voltage exceeds the capacitor voltage. These pulses of current are of the fundamental 60 hertz frequency (60 Hz) but are rich in 3rd harmonic components and usually also contain significant 5th and 7th harmonics. The harmonic content varies with circuit values such as the size of the capacitor. In a three-phase four-wire system, the 3rd harmonic current and its multiples from each phase add in the neutral conductor even with balanced loads whereas the fundamental and the other harmonic currents cancel in the neutral conductor. In the worst case when 100% of the rated current is flowing in each phase, 173% of the rated current is flowing in the neutral conductor when only this rectified form of load is connected.
The existence of this problem is well known in the computer industry and efforts to advise customers of it are underway through the Computer and Business Equipment Manufacturers Association (CBEMA). CBEMA recommends, among other things, installation of an over current detector in the neutral conductor which will trip an upstream circuit breaker to protect the neutral conductor from overheating if it is not feasible to oversize the neutral conductor. They also recommend a periodic inspection of the electrical system to detect overload and/or overtemperature.
This creates a dilemma for an office building manager who may never have an overload or unsafe condition. On the one hand, it is not attractive to upgrade a power distribution system, potentially at great expense, for a problem which may never occur. On the other hand, periodic testing which consistently shows a properly loaded power distribution system is likely to be discontinued as an unnecessary expense.
Even if testing is performed on a periodic basis, it may not detect a problem in time to prevent overheating with potential fire hazard and equipment damage. Further, an unscheduled shutdown of power, especially to the office automation equipment causing the problem, may well be disruptive and possibly expensive for the users even though protection, such as circuit tripping, is necessary for safety. In addition, office automation equipment can even be effected if no shut down occurs. Harmonic neutral circuit overloads can result in loosening of connections in the neutral circuit which, in combination with neutral currents, can produce common mode neutral to ground noise in the power system leading to data errors in the office automation equipment.
Thus, there is a need for an inexpensive and reliable neutral circuit monitoring device for three-phase four-wire electric power distribution systems which detects harmonic currents flowing in neutral circuits and warns operators of the power distribution systems of an overload condition before the occurrence of damage to the power distribution systems or problems in equipment powered by the systems.